This invention relates to a lens array, an exposure device, an image forming apparatus and a reading apparatus.
There is known an image forming apparatus (such as a printer, a copier, a compound machine or the like) having an array of LEDs as light emitting portions. Such an image forming apparatus uses an array of rod lenses disposed in opposition to the light emitting portions. The rod lenses are configured to focus the lights emitted by the light emitting portions onto a surface of a photosensitive drum.
Moreover, there is knows a reading device (such as a scanner) having an array of light receiving portions. Such a reading device uses an array of rod lenses disposed in opposition to the light receiving portions.
The rod lens is an optical element formed of a glass fiber impregnated with ion so that the refractive index decreases from the center portion toward the peripheral portion. The rod lens forms an erected image at the same magnification. A lens array in which rod lenses are arranged in a plurality of rows is used as an optical system for focusing an image of an object as a linear image.
As an example of a simple optical system, there is known a lens array in which rod lenses are arranged in a single row. Further, there is proposed another lens array including lens-pairs each of which includes a pair of micro lenses disposed at an interval corresponding to the focal length so that optical axes thereof are aligned with each other (see, for example, Japanese Laid-Open Patent Publication No. 2002-107661). The lens array using the micro lenses has the same function as the lens array using the rod lenses.
However, in the conventional lens array using the rod lenses, there is a difference in optical properties between the vicinity of the optical axis of each rod lens and the periphery of each rod lens, and therefore the degradation of the resolution and the reduction of the amount of light may occur with a cycle corresponding to the interval at which the rod lenses are arranged. Similarly, in the lens array using the micro lenses, there is a difference in optical properties between the vicinity of the optical axis of each micro lens and the periphery of each micro lens, and therefore the degradation of the resolution and the reduction of the amount of light may occur with a cycle corresponding to the interval at which the micro lenses are arranged.
In this regard, there is proposed another lens array in which micro lenses are arranged in a staggered manner (see, for example, Japanese Laid-Open Patent Publication No. 2000-221445). It is possible to arrange such lens arrays in the direction of the optical axis at an interval corresponding to the focal length. However, in such a case, the amount of light may be reduced as the resolution is enhanced.